For widespread acceptance, photovoltaic cells (solar cells) typically need to produce electricity at a cost that competes with that of fossil fuels. In order to lower these costs, solar cells preferably have low materials and fabrications costs coupled with increased light-to-electric conversion efficiency.
Thin films have intrinsically low materials costs since the amount of material in the thin (˜2-4 micrometer (μm)) active layer is small. Thus, there have been considerable efforts to develop high-efficiency thin-film solar cells. Of the various materials studied, chalcopyrite-based devices (Cu(In &/or Ga)(Se &, optionally S)2, referred to herein generically as “CIGS”) have shown great promise and have received considerable interest. The band gaps of CuInS2 (1.5 eV) and CuInSe2 (1.1 eV) are well matched to the solar spectrum, hence photovoltaic devices based on these materials are expected to be efficient. To date, thin-film CuInS2 cells with efficiencies of 12.5% and Cu(Inga)Se2 cells with efficiencies of 18% have been recorded.
Conventional fabrication methods for CIGS thin films involve costly vapour phase or evaporation techniques. However it is possible to form thin films by melting or fusing nanoparticulate material into a thin film such that the nanoparticles coalesce to form large-grained thin films. This may be done using metal oxide nanoparticles followed by reduction with H2 and then by a reactive sintering step with a selenium containing gas, usually H2Se. Alternatively and preferably, this may be done using prefabricated CIGS nanoparticles. The use of CIGS nanoparticles avoids the hazardous reduction of metal oxides with H2 and selenization with toxic H2Se.
To use nanoparticulate CIGS-type nanoparticles (i.e., CIGS or similar materials) as a starting point to form the large grain thin film, the CIGS-type nanoparticles preferably have some key properties. Firstly, the nanoparticles are preferably small. When the dimensions of nanoparticles become smaller their physical, electronic and optical properties might change. The restriction of an electronic wavefunction to smaller and smaller dimensions within a particle is referred to as “quantum confinement”. When nanoparticles are in this size regime they are often referred to as quantum dots. It is known that as the size of nanoparticles decreases their corresponding melting point also decreases. Also, smaller particles typically pack more closely, which may help the coalescence of the particles upon melting. Secondly, a narrow size distribution is important. The melting point of the nanoparticles is related to their size; a narrow size distribution means that all of the particles typically melt at approximately the same temperature, giving an even, high quality (even distribution, good electrical properties) film. Finally, a volatile capping agent for the nanoparticles is generally preferred so that, upon relatively moderate heating, the capping agent may be removed to reduce the likelihood of carbon or other elements contaminating the final film upon melting of the nanoparticles.
A number of methods are known for preparing CIGS-type nanomaterials. These include multi-component colloidal methods, single-source precursor methods, and solvo-thermal methods. Except for the case of single-source precursor methods, known methods generally involve the reaction of Cu, In and Ga halides with a source of selenium. Typically it is elemental selenium or selenium coordinated by trioctylphosphine (TOP); however reaction with H2Se has been demonstrated.
Reaction of Cu(I), In(III), Ga(III) salts in water or acetonitrile with bubbling H2Se or H2S with 1% polyvinyl alcohol to act as a stabilizer led to colloidal nanoparticles of the corresponding CIGS material. Sizes of the prepared nanoparticles ranged from 2-6 nm in water and 1-4 nm in acetonitrile. (See Gurin, Colloids Surf. A 1998, 142, 35-40.)
Reaction of CuI, InI3, GaI3 in pyridine with Na2Se in methanol at reduced temperature under inert atmosphere yielded pyridine-capped Cu(In, Ga)Sex nanoparticles where the relative amounts of the elements in the product reflected the amounts in the precursor solutions. The product material was amorphous so the phase composition could not be ascertained; residues of samples heated to 400° C. for thermal gravitation analysis (TGA), however, showed pure CIGS phase formation. Overall size distributions determined from transmission electron microscopy (TEM) micrographs were estimated at 10-30 nm and the shape of the nanoparticles changed from spherical to tube-like depending on the Cu/(In, Ga) ratios. (See Shulz et al., J. Elect. Mat. 1998, 27, 433-437; Kim et al. J. Mech. Sci. Tech, 2005, 19, 2085-2090.)
Reaction of InCl3, CuCl and TOPSe in tri-octylphosphine oxide (TOPO) at elevated temperatures (˜250° C.) resulted in TOPO-capped CuInSe2 nanoparticles of approximately 5 nm in size. Increasing the temperature of reaction to 330° C. yielded particles 15-20 nm in size. (See Malik et al. Adv. Mat. 1999, 11, 1441-1444; Arici et al. Thin Solid Films, 2004, 451-452, 612-618.)
Reaction of CuCl, InCl3 with dodecanethiol followed by addition of octadecene and TOPSe at a temperature of 180° C. resulted in dodecanethiol-capped CuInSe2 nanomaterials ranging in size from 6 nm nanoparticles to 200 nm nanoplates depending reaction time. These nanoparticles displayed size-dependent quantum confinement behavior in the absorption spectra but, critically, no photoluminescence was observed. The authors noted that use of other capping agents such as TOPO, hexadecylamine, oleylamine or a combination of these ligands did not lead to formation of CuInSe2 which matches the inventors' own experiences. (See Zhong et al. Nanotechnology, 2007, 18, 025602.)
Single-source precursor molecules have also been investigated for the preparation of colloidal CIGS-type nanoparticles. Thermal decomposition of the single source precursors (PPh3)2CuIn(SEt)4 and (PPh3)2CuIn(SePh)4 in dioctylphthalate at temperature between 200-300° C. led to the formation of nanoparticles of CuInS2 and CuInSe2 respectively. The nanoparticles produced ranged in size from 3-30 nm and were spherical in shape. Because of the use of non-coordinating solvents the nanoparticles formed agglomerates of insoluble material. (See Castro et al. Chem. Mater. 2003, 15, 3142-3147.)
Similar single source precursors (TOP)2CuIn(SR)4, where R=i-Pr or t-Bu, have been decomposed photolytically in dioctylphthalate to produce small (˜2 nm) organic soluble CuInS2 nanoparticles. (See J. J. Nairn et al. Nano Letters, 2006, 6, 1218-1223.)
Solvothermal methods that may be carried out at relatively low temperatures and do not require organometallic or toxic precursors have been investigated for the preparation of CIGS nanomaterials. Typically copper halide, indium halide, gallium halide, selenium and/or sulfur are added to a non-stick coated autoclave, which is filled with a solvent such as benzene, ethylene diamine or water, sealed and then heated for period of time. This process provided nanomaterials having an elemental composition that reflected the stoichiometries of the reactants and morphologies determined by the interaction of the solvent during the reaction. (See Li et al. Adv. Mat. 1999, 11, 1456-1459; Lu et al. Inorg. Chem. 2000, 39, 1606-1607; Jiang et al. inorg. Chem. 2000, 39, 2964-2965; Xiao et al. J. Mater. Chem. 2001, 11, 1417-1420; Hu et al. Sol. State Comm. 2002, 121, 493-496; Chun et al. Thin Solid Films 2005, 480-481, 46-49; Gou et al. J. Am. Chem. Soc. 2006, 128, 7222-7229). For example, reaction of stoichiometric amounts of CuCl2, InCl3, Se in ethylenediamine or diethylamine in a sealed autoclave at 180° C. for 15-36 hours yielded relatively large CuInSe2 nanowhiskers (width 3-6 nm, length 30-80 nm) and nanoparticles (diameter 15 nm) respectively. The elemental ratios in these materials closely matched the stoichiometries of the reactants. (See Li et al. Adv. Mat. 1999, 11, 1456-1459.)
Similarly, nanocrystalline CuInS2 and CuGaS2 have been prepared solvo-thermally by reaction of CuCl, In or Ga, and excess S in benzene or water in a sealed autoclave heated at 200° C. for 12 hours. The sizes of the CuInS2 and CuGaS2 nanoparticles were 5-15 nm and 35 nm respectively. (See Lu et al. Inorg. Chem. 2000, 39, 1606-1607.)